Multiple transmit and receive antennas can greatly increase the efficiency of wireless communication systems, E. Telatar, “Capacity of multi-antenna Gaussian channels,” European Transactions on Telecommunications, vol. 10, pp. 585-595, November-December 1999, G. J. Foschini and M. J. Gans, “On the limits of wireless communications in a fading environment when using multiple antennas,” Wireless Personal Communication, vol. 6, pp. 315-335, March 1998. When channel state information (CSI) is known at the transmitter in a MIMO system, singular value decomposition (SVD) transmission with water-filling can be used to approach channel capacity, see Telatar.
However, channel estimation error and CSI feedback delay in time-varying channels cause CSI ambiguity at the transmitter. The loss of orthogonality between virtual channels formed by SVD causes mutual interference and significantly degrades system performance.
For many systems, perfect instantaneous CSI at the transmitter (CSIT) is not possible. For frequency division duplex (FDD) systems, in which the forward and the reverse links operate at different frequencies, instantaneous CSIT would require continuous fast feedback, possibly in a separate feedback channel, which decreases the spectral efficiency. For time division duplex (TDD) systems, in which the forward and reverse links operate at the same frequency, instantaneous CSIT is impossible, especially if the system operates in channels with small coherence intervals. Perfect CSIT requires zero delays between the forward and reverse links.
Prior art MIMO systems provide feedback of the CSI at a fixed rate, e.g., 3 GPP provides channel state feedback every 10 ms. It is desired to provide the CSI at a minimum rate, while still meeting predetermined performance criteria.